Wild Country Friends - Could you explain this ?

Today I watched this movie from the Wild Country website
vimeo.com/169105191
... and at 1:10 I noticed a strange elastic behaviour of the green .75 friend: while catching the fall, the cams open a little than revert to the initial position. I see no other reason for this but an elastic deformation of the (new hollow) axles and the amplitude of the movement seems quite huge, which for me would be a deal breaker.

I wanted to add some of these friends to my rack, so somebody please prove me wrong ! :)

Looks like a video of a cam working perfectly fine to me.

The cam clearly did not break, deform, or yield permanently (plastic deformation), so I do not see what the big deal is. I would not be surprised to see a little elastic deformation of a part designed to be lightweight, and to hold a certain about of load in a fall + safety factor.

Once the force was no longer applied, the cam returned to it's original shape. Seems perfectly normal.

I think it's because the cam is in a bit of a flare. I can't imagine that deformation being elastic if it's actually caused by the axles bending.

However, why is elastic deformation an issue for you? It won't cause any permanent damage to the cam, by definition.

Go aid climbing and you'll see cams doing this exact same thing all the time. Perfectly normal.

Alexander Blum wrote: However, why is elastic deformation an issue for you? It won't cause any permanent damage to the cam, by definition.

Well, this one was caused by a small fall. Knowing that the elastic deformation is proportional with the force, how will it behave in a large fall, with greater forces involved ?

And I dont't like my cams moving under load. In the movie, you can see the cams are rocking on the crack walls. (It's true, they dont slide...)

Here's a possible explanation: It looks like the point of contact of the top lobe with the rock is right next to one of the grooves on the lobe surface (the ones that are ostensibly meant to help the lobes grip better). When the cam is weighted, the point of contact rolls over the groove (so now the lobe is contacting the rock on either side of the groove), allowing the cam to open more than normal.

Obviously a small amount of elastic deformation is necessary to allow this rolling to happen in the first place, but I think the surprising amount of movement here is more due to the unusually large grooves in the cam lobes, rather than an unusual amount of elastic deformation.

That being said, I've never really seen a close up of a cam subjected to fall forces before, so maybe this is a normal amount of elastic deformation, and no alternative explanation is necessary.

I would almost gamble that the new DMM dragons or the ultralight camalots would behave the same way placed in a flaring crack. Maybe watch some of the cam testing videos for how they behave you might see some of the same behavior at normal "working" loads.

Verticon, I would say that it will stay elastic at anything less than the rated strength of the cam. As would be expected.

I would also point out what Trevor just said - this probably happens to some degree every time a cam is weighted (just tested it in my garage). This happens, at least partially, because the spring force is many orders of magnitude less than the normal force that is exerted once the cam is fallen on.

Trevor. wrote:Go aid climbing and you'll see cams doing this exact same thing all the time. Perfectly normal.

i can back this up. i see this all the time on my C and X4s when aid climbing. the spring forcing the cam open and holding it in place doesnt have nearly the force of body weight or a fall.

i feel like everyone should do a couple pitches of clean aid climbing to get a good insight into their gear placements. i think people would learn a lot about small cam placements especially.

at a guess, we are looking at

- no load
- initial high load, rope pulls tight, pulls cam as far open as crack will allow
- rope begins to stretch decreasing the load and
- recoil upward, as the initial high force and load on the rope is
reduced to the climber's static weight on the rope

- seems to me that the springs in cams hold the cam in the
crack, but don't force it as far open as it can go
- seems like a good reason not to tip out your cams in a
placement
- also seems like it's time to review other video for
comparison

the only thing i saw that i found weird was, why the whole hanging belay thing? ughh, just have her belay from the ground instead of getting the whole sideways mashup thing every time he falls. must be for the cinematography.

as for the cams, not sure why you find that so unsettling?

slim wrote:as for the cams, not sure why you find that so unsettling?

I had a full set of Helium Friends and I didn't see them (and neither my other cams, DMM Dragons and Metolius Master Cams) deforming like this (or maybe it wasn't enough give to be noticed under body weigth while aiding ?!)

When I saw the movie, I considered the following hypotheses:

Once the cams are touching the rock there's no room for the device to expand any further.

If the rock doesn't break at the contact points and the cams don't deform but they are still expanding under load, the give could come only from the cam-axle assembly.

I guess I'm just suspicious because of the new hollow axle. I think it's strange for the designer to weaken the most stressed part of the device...

It's a design tradeoff. As long as the cam is strong enough (I can't imagine it would pass CE/UIAA testing I'd this was a valid concern), and this tradeoff makes the cam lighter, I just don't see a cause for concern. The one possibility (if we are see elastic deformation in that video ) would be fatigue strength. If the axle is steel, this likely would not be an issue until upwards of 10,000-20,000 cycles.

verticon wrote: I guess I'm just suspicious because of the new hollow axle. I think it's strange for the designer to weaken the most stressed part of the device...

FYI, for a cylindrical beam (such as an axle), the material on the axis provides zero support, and the material near the center is not nearly as important as the material near the outside. For example, a solid tube is less than 10% stronger/stiffer than a hollow tube with the inside diameter half the outside.

en.wikipedia.org/wiki/List_…

(This is not true for pure shear strength, which is just a function of area).

Cams are all UIAA tested and approved, so you know that this cam is strong enough for anything you will throw at it. Don't worry about it.

Alexander Blum wrote:The one possibility (if we are see elastic deformation in that video ) would be fatigue strength. If the axle is steel, this likely would not be an issue until upwards of 10,000-20,000 cycles.

It's pretty easy to design a steel member for infinite fatigue life, and given the safety factors on climbing gear, I would not be surprised if this was the case.

Kyle, I agree with that. When I posted that number I just looked at the old BD QC lab post on crampons, where they did some fatigue tests. The lowest cycles to failure were around 10,000 and most were well above that. My point, refined, would be that fatigue life is probably a non-issue, but even if it was a cam simply won't see a number of cycles like that in its lifetime. That would be three hard falls every single day for ten years, at a minimum.

There is no question of whether elastic deformation occurs when a cam is loading. It does occur. This is engineering 101 and is expected for any loading.

Nerds......

verticon wrote: I had a full set of Helium Friends and I didn't see them (and neither my other cams, DMM Dragons and Metolius Master Cams) deforming like this (or maybe it wasn't enough give to be noticed under body weigth while aiding ?!) When I saw the movie, I considered the following hypotheses: Once the cams are touching the rock there's no room for the device to expand any further. If the rock doesn't break at the contact points and the cams don't deform but they are still expanding under load, the give could come only from the cam-axle assembly. I guess I'm just suspicious because of the new hollow axle. I think it's strange for the designer to weaken the most stressed part of the device...

1) you must not have been paying that close of attention - the cams will always rotate a bit under load.

2) why are you assuming the hollow axle is weaker? why are you assuming it is the most stressed part?

If you look at the video you can see him swinging back and forth, I think what you are seeing is the cam pivot/rock forward in the flare slightly until it catches and then as he flails in space the rope swings left and the weight shifts on to the purple cam (maybe mixed in with his recoil upwards) and the green cam relaxes back into the position the springs had initially kept it in.

slim wrote: 1) you must not have been paying that close of attention - the cams will always rotate a bit under load. 2) why are you assuming the hollow axle is weaker? why are you assuming it is the most stressed part?

1) I agree, the cams will rotate a bit under load, until they set (some normal play between the cams and the axles, a crystal might break, the rock surface might crush a little bit etc.). But then, when you remove the load, there's no force in the system to pull them back in their initial position. What makes the cam in the movie revert to the initial position ?

2) You just try to draw a 3D stress diagram for such an axle and you will see :)

I'm not worried about the cam breaking on me, I'm sure it won't (it's a Wild Country Friend after all...)

The problem would be the stability of the cam. This type of movement of the cams on the crack walls under load is quite unpredictible and I think it might cause the cam to pull.

Another effect of the elastic deformation of the axle: if some of the force pressing the cam on the rock is lost on bending the axle, the friction would decrease, causing the cam to pull easyer

So, how predictible is the movement ? Could it cause the cam to pull ?

It just feels like you're armchair/monday morning quarterbacking the engineers who designed this cam with a bunch of minutia. Yes, a hollow axle will be weaker than a solid one, all things being equal. That doesn't make any of your subsequent concerns valid, though. It's like you think they didn't test their product at all before releasing it.